Vehicle-heating system employing a critical point heat pipe

ABSTRACT

The employment of a heat pipe as a passive, self-operating thermal energy transport valve which automatically thermally couples a heat source to a heat sink or decouples the same depending upon the operating temperature of the heat pipe and its relation to the critical point of the working fluid. The critical point heat pipe can serve as an auxiliary heat transport device for supplying heat from the exhaust gases of a vehicle engine to the vehicle compartment heat exchanger during engine warmup.

United States Patent Re.l8,l10 6/1931 Sidney Frank Baltimore, Md. 786,053

Dec. 23, 1968 Nov. 9, I971 Isotopes, Inc. Westwood, NJ.

Inventor Appl. No. Filed Patented Assignee VEHICLE-HEATING SYSTEM EMPLOYING A CRITICAL POINT I-IEAT PIPE 9 Claims, 6 Drawing Figs.

US. Cl 237/l2.3 B, 165/105 Int. Cl B60h 1/08 Field of Search 237/123 W, 2; 165/32,

References Cited UNITED STATES PATENTS Vernet 237/12.3 W

Vernet 237/123 W X Hans 237/12.3WX Chausson. 237/2 A X Swet /32 Fischell 165/32 Shlosinger 165/32 Primary ExaminerEdward J. Michael Attorney-FleiLGippleg; Jacobson ABSTRACT: The employment ofa heat pipe as a passive, selfoperating thermal energy transport valve which automatically thermally couples a heat source to a heat sink or decouples the same depending upon the operating temperature of the heat pipe and its relation to the critical point of the working fluid.

The critical point heat pipe can serve as an auxiliary heat transport device for supplying heat from the exhaust gases ofa vehicle engine to the vehicle compartment heat exchanger during engine warmup.

PAIENTEDuuv 9 I97! SHEET 1 [IF 3 INVENTOR SIDNEY FRANK ATTOR NEYJ,

PATENTEDNUV 9197i 3. 6 l 8 854 SHEET 3 0F 3 LEGENDS 3 H hot (source) 7 R refererrce C cold (slnk) AMB ambrent V vapor (heat pipe) E evaporator Iv, CR critical T threshold TH I TC HEAT PIPE SOURCE SINK FIG. 4 TCR: TR

3; E [T INCREASING O: LLI Q- I LLJ r- 'OFF 'FULL OFF TIME FIG. 5 T

X FIG. 6 E

2 J T I 3 T (2R D E E E INVENTOR M SIDNEY FRANK ATTORNEY-5:

VEHICLE-HEATING SYSTEM EMPLOYING A CRITICAL POINT HEAT PIPE Within recent years there has come into vogue a simplified, essentially isothermal device for transporting heat in an expeditious and efficient manner. The device, which is known as a heat pipe, is, in its simplest form, a closed container, normally metallic, employing on the inner surface a capillary structure or wick which is essentially saturated with the liquid phase of a working fluid. The heat pipe transfers heat, almost isothermally, from one point on the external surface to any other point by a vaporization-condensation cycle. The capillary structure may be comprised of grooves, single or multiple layers of wire screen, or any other suitable system of capillaries to move the liquid-phase-working fluid from the condenser or heat sink of the heat pipe to the evaporator orheat source.

A measure of the capacity of a working fluid to transport a large quantity of heat in a heat pipe is given by the liquid transport factor, N,,, which is a property of the fluid, has units of heat flux (e.g. watts per square centimeter), and can be expressed by the formula:

arepresents the surface tension,

A represents the heat of vaporization,

P represents the liquid density, and

[LL represents the dynamic viscosity of the liquid.

Thus, for general heat pipe applications, it is desirable to have the liquid transport factor N, of the heat-pipe-working fluid as high as possible and, in most cases, the working fluid for the heat pipe is selected on the basis of its liquid transport factor when there are two or more fluids whose liquid-range (temperature range from triple point to critical point) includes the desired operating temperature.

A vapor transport factor, N,,, can be formed in a manner similar to that for N, by using the density and viscosity of the vapor rather than those of the liquid. When curves'of N and N,, are plotted against temperature for a particular working fluid, each exhibits a single maximum value. The temperature corresponding to the maximum N (T,,*) is lower than the temperature corresponding to maximum N, (T,,*) while the value of N maximum is greater than that of N, maximum. Both curves merge and fall to zero at the critical point.

FIG. 2 is a logarithmic plot of liquid and vapor transport factors N L and N,,, respectively, for three typical vaporizable working fluids. Curve a is a plot of vapor transport factor N, for ammonia (NH,), while the curve a is the liquid transport curve for the same working fluid. In like fashion, curve b is a plot of the vapor transport factor N for methanol, curve b is the plot of the liquid transport factor N, for the same composition. Curves c and c show plots of vapor transport factor N, against temperature for water and liquid transport factor N L against temperature.

Thus, by referring to FIG. 2, it is shown that the critical point temperature for ammonia is just above 260 F., for methanol, it is above 440 F., and for water is above 700 F. It can be shown both experimentally and analytically by a procedure which will not be demonstrated here that an optimum temperature (T"') exists for a given heat pipe and working fluid at which the heat transport capability is a maximum and that:

The working fluid for the heat pipe operates very near the equilibrium condition between liquid and vapor. The capillary action in the wick causes the liquid phase to move from the condenser or heat-rejecting surface of the heat pipe to the evaporator or heat receiving surface of the heat pipe. The gaseous phase moves in an opposite direction within the central void region of the heat pipe, after its formation at the evaporator. The isothermal heat transport phenomenon persists as the operating temperature is increased until the optimum temperature, T", is exceeded and the critical temperature, T of the working fluid is approached. Since both the heat of vaporization and the surface tension decrease with increasing temperature until they become zero at the critical temperature, a threshold temperature, T less than T but greater than T* exists beyond which heat transported by the heat pipe diminishes. When the threshold temperature is reached the evaporator begins to dry out andwhen the critical temperature is reached, the evaporator is completely dried out. The greater the rate of heat transport through the functioning heat pipe, the lower the threshold temperature.

Reference to FIG. 3 shows schematically a heat pipe positioned between a source of thermal energy at temperature T and a heat sink at temperature T the temperature at the source being hotter than the temperature at the sink which is relatively cold. The discussion set forth in the preceding paragraph may be seen visually by reference to FIG. 4 which shows the operation of a critical point heat pipe in a typical environment, such as that of FIG. 3, with'ambient temperature increasing. The plot shows temperature against time with the heat source delivering thennal energy to the heat sink via the heat pipe which employs a vaporizable working fluid having a critical point temperature equal to a reference temperature. Initially, the heat pipe is operating to transport heat isothermally from the heat source to the heat sink and this operation continues as the ambient temperature increases until a time r at which point threshold temperature T is reached, which is less than T but greater than T", at which point the heat transport from the heat source to the heat sink starts to diminish. When the threshold temperature I} is reached, the evaporator starts to dry out and is completely dried out at T this occurring at a time t when the heat pipe is no longer functioning to transport heat from the source to the sink.

This action is a function of the evaporator heat flux which is related to the length of wetted surface present in the evaporator and is further demonstrated in FIG. 5. Initially, with heat flux plotted against temperature, there is a steady, uniform passage of heat from the source to the heat sink until the temperature T is reached, whereupon the heat flux decreases and at the critical point T heat flow ceases.

Reference to FIG. 6 shows the opposite effect in a decreasing ambient temperature situation where a control function is initiated probably after the temperature decreases somewhat below the critical temperature T because of anticipated hysteresis in the system. Heat is isothermally transported from the source to the heat sink when the threshold temperature T, is reached. At that point, as is indicated at time t,,,,, thermal energy is transported at an increasingly greater rate until time tFULL which corresponds to the desired reference temperature T,, for the heat sink. At that time, the heat pipe is delivering sufficient thermal energy to maintain the heat sink temperature T at a constant reference temperature T From the above discussion, it is apparent that, unlike previous heat pipe applications in which it has been the practice to select a working fluid having the highest possible liquid transport capability, when control" is an objective rather than heat transfer or transport" alone, the critical point temperature T in relationship to the desired reference temperature becomes the criterion for selection of the heat-pipe-working fluid. In the selection of a working fluid for a conventional heat pipe system, the critical point may not be within or near the operating range of the system, otherwise the heat transport capability would be extremely poor. Where the heat pipe is to perform a control function the critical point must be within the systems operating range since approaching the critical temperature from below tends to turn off" the low of heat and approaching the critical temperature from above tends to turn on the flow of heat. While the single most important criterion for control resides in the critical temperature of the working fluid, the selection of the same may result in a containment problem. Where two or more fluids have critical temperatures in the same range, the one with the lowest critical pressure readily distinguishes between them. For example, in considering the use of freons, such as F reon-l 2 and Freon-22, the critical temperature of Freon-l 2 is 233 F., and while its critical pressure is 5 p.s.i.a. For F reon-22, the critical temperature is slightly less at 205 F., but a remarkably increased critical pressure of p.s.i.a. of 720 p.s.i.a. exists. Thus, from a pressure containment standpoint, Freon-l2 is a better selection than F reon-22 in control heat pipe applications in the temperature range from 200 to 250 F.

Automotive vehicle cabs or passenger compartments are heated normally by diverting part of the engine coolant to a heat exchanger located within an air circulation or recirculation duct and normally positioned between the engine and the compartment or cab. There is an appreciable time lag between the instant of starting the vehicle engine and the time at which the temperature of the coolant is high enough to sufficiently heat the compartment air circulating through the heat exchanger. When the vehicle is operating in an extremely low temperature outside air environment, this time lag may extend over several minutes with considerable discomfort to the operator of the vehicle or other occupants of the cab. While the utilization of waste heat present in the engine liquid coolant is an economical method of heating the vehicle cab interior, there have been no real solutions to the problem created by the time lag during which the coolant is attaining its normal operating temperature.

It is, therefore, a primary object of this invention to provide an improved self-operating thermal energy transport valve which thermally couples a heat source to a heat sink and automatically decouples the same when the system threshold temperature is reached.

It is a further object of this invention to provide an improved method of maintaining the temperature of a heat sink above a predetermined level relative to a high-temperature heat source which is thermally coupled thereto by a heat pipe. temperature It is a further object of this invention to provide an improved vehicle heater system in which the temperature of the vehicle compartment may be rapidly raised subsequent to engine starting.

It is a further object of this invention to provide an improved vehicle-heating system of the type in which the auxiliary heat transport function is achieved by heat pipe means whose function automatically ceases when the temperature of the vehicle engine coolant reaches normal operating temperature.

Other objects of this invention will be pointed out in the following detailed description and claims and illustrated in the accompanying drawing, which discloses, by way of example, the principle of the invention and the best mode which has been contemplated of applying that principle.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially schematic, perspective view of the vehicle-heating system employing a series of critical point heat pipes of the present invention.

FIG. 2 is a plot of liquid and vapor transport factors against temperature for various heatpipe-working fluids.

FIG. 3 is a schematic representation of a control heat pipe for controllably transporting heat isothermally from a heat source to a heat sink.

FIG. 4 is a plot of temperature against time for the heat pipe application of FIG. 3 wherein heat transport ceases as the heat sink temperature reaches the reference temperature.

FIG. 5 is a plot of the evaporator heat flux against temperature for the control heat pipe application of FIGS. 3 and 4.

FIG. 6 is a plot of temperature against time for a modified critical point heat pipe control system of FIG. 3 in which isothermal heat transport is initiated in response to reduction in system temperature.

In general, the present invention is directed broadly to the method of raising a heat sink to a desired threshold temperature and maintaining it above that level regardless of variations in availability of other heat sources or of other paths of heat transfer to the heat sink. The method involves thermally coupling the heat sink to a heat source by heat pipe means and providing a heat-pipe-working fluid having a critical temperature slightly greater than the desired threshold temperature.

This method is applied to a conventional vehicle-heating system employing engine coolant for heating air circulated within the vehicle passenger compartment. In addition to the engine coolant heat exchanger, there are provided one or more heat pipes having their condenser ends within the same air duct and their evaporator ends operatively coupled to the exhaust manifold of the engine. The heat pipes contain a working fluid having a critical temperature which exceeds the desired threshold temperature for heat pipe shutoff whereby during warmup thermal energy is delivered to the compartment circulating air principally by the heat pipes until the engine coolant reaches its operating temperature. The heat pipes automatically begin to shut off when the heat-pipe-operating temperature reaches the threshold temperature and completely shut off when the critical temperature is reached. Preferably, the evaporator ends of the heat pipes are lower in elevation than the condenser ends in contact with the exhaust manifold.

In one specific form, the present invention is directed to the employment of one or more heat pipes having their evaporator ends operatively coupled to the vehicle engine exhaust manifold while their condenser ends are operatively coupled to the compartment air circulating duct. During engine warmup, heat can then be transferred from the moment of engine start in an essentially isothermal manner between the engine exhaust manifold which acts as an auxiliary heat source and the heater air duct. These heat pipes form self-operating valves which insure rapid transfer of heat from the exhaust gases during engine warmup and then terminate heat transfer once the engine coolant reaches its operating temperature, so as not to adversely affect the normal vehicle-compartmentheating operation.

Turning to FIG. 1 of the drawing, a conventional automotive vehicle 10 of the passenger type is provided with a vehicle passenger-compartmenbheating system of a conventional nature which is mounted beneath the dashboard and is oriented between the engine block 12 and the front of the passenger compartment indicated by area 14. The line 16 normally defines the base of the windshield and separates the compartment interior 14 from the outside environment. Conventionally, just in front of the windshield (not shown), a number of air intake ports or slots 18 are formed within the sheet metal cowl 19 with fresh air entering these slots as indicated by arrow 20. The air enters the heat exchanger assembly 22 through a cylindrical duct 24 at the right-hand end of the passenger compartment 14. The heater assembly 22 is shown partially cut away to indicate the location of the principal components including heat exchanger 26, temperature controller 28, blower 30, defroster control mechanism 32, compartment air discharge duct 34 and defroster air discharge ducts 36 and 38.

Essentially, for the purposes of the present invention, it is assumed that the fresh air entering duct 24 at the right-hand end of the heater assembly, immediately passes through the heat exchanger 26, as indicated by arrow 40, or bypasses the heat exchanger as indicated by arrow 42. The amount of air which is heated, as against the bypass air, is controlled by the pivotable temperature door 28 which is normally manually adjusted to provide the desired temperature to the compartment air delivered by discharge duct 34 as indicated by arrow 44. This air first passes through blower 30. With the shutoff door 46 in the position shown, the air is directed as indicated by arrow 48 into the heater discharge duct 34. Of course, by clos ing the shutoff door 46, air will pass into defroster chamber 50, and exit through circular ports 52 to the paired defroster ducts 36 and 38.

Conventionally, a plate-and-tin-type heat exchanger 26, which is shown as diagonally oriented within heat exchanger chamber 54, is operatively coupled to the engine block and receives engine coolant through engine coolant inlet 56. The liquid engine coolant is returned to the engine by engine coolant return duct or tube 58 on the opposite side of heat exchanger 26. In this respect, the engine block is shown schematically and the inlet and return ducts coupling the engine to the fin and tube heat exchanger 26 are shown in line form only. Obviously, suitable controls, such as thermostats are needed to regulate the movement of coolant between the engine block 12 and the heat exchanger 26.

conventionally, therefore, once the engine is started, the coolant heats up gradually and as the coolant increases in temperature, more and more heat is transferred by heat exchanger 26 to the air circulating through the heat exchanger assembly 22 at a velocity, depending upon vehicle speed and blower speed if the blower motor is operated. The problem, as stated previously, resides in the fact that there is a time lag between the instant of starting and the instant when the engine coolant has increased in temperature enough to pass sufficient heat to the air entering heat exchanger assembly 22. In extremely cold weather, this time lag may be several minutes, even though the engine is operating fully during this period, and, while the exhaust gases of the engine are relatively hot, the heat of exhaust is not readily available to the occupant of the vehicle.

In the embodiment shown, three heat pipes 60 have their evaporator ends 62 positioned within exhaust manifold 64 which is also shown schematically in block form as being physically coupledto and incorporated within engine block 12. The condenser ends 66 are positioned within heat exchanger compartment 54 and, oriented vertically in spaced fashion, adjacent to the downstream side of the plate and tin heat exchanger 26 whereby, the fresh air entering duct 24 may be rapidly heated during the engine warmup period. Preferably, some means 68 of insulating the lengths of heat pipes which extend between the evaporator sections 62 and the condenser sections 66 is employed to prevent heat loss.

ln all respects, except one, the heat pipes 60 are conventional and include, capillary liquid transport means (not shown) internally of the pipes for transporting liquid phase working fluid from the condenser sections 66 to the evaporator sections 62. Also, void spaces are provided within the heat pipes for transporting gaseous-phase-working fluid in the opposite direction from the evaporator sections 62 to the condenser sections 66. The essential diflerence between the heat pipes employed in the present invention and conventional heat pipes is the fact that, unlike prior heat pipe applications, the property of the fluid whereby its liquid and vapor transport factors decrease with increasing temperature as the critical point is approached, is utilized to achieve thermal control. Since function of the heat pipes 60 after the engine warmup interval would disrupt the normal heater operation, it is necessary to prevent transport of heat by the auxiliary means after the plate-and-fin-type heat exchanger 26 is operating adequately.

The present invention is directed both in its broadest form and in the specific environment of an automotive vehicle heat exchange system to the concept of correlating the critical temperature of the vaporizable working fluid within the heat pipe to a threshold temperature of the system at which it is desirable either to begin transporting heat, in the case of falling temperature, or to reduce the transport of heat, in the case of rising temperature. The threshold temperature, or heat pipe control point, is in either case less than the critical temperature but greater than the heat pipe optimum temperature T* as defined above. Thus, in the illustrated environment it is preferable to employ freon of a type which, while not having a particularly good N,,, has a critical temperature in the vicinity of 200 F., at which temperature the heat pipe it to stop delivering heat to the compartment air. In particular, Freon-l2 with a critical temperature of 233 F. and a critical pressure of 582 p.s.i.a. would be suitable. F reon-22 which has a critical temperature of 205 F. could also be considered but its critical pressure of 716 p.s.i.a. makes it less desirable than Freon-l2 from the standpoint of pressure containment. Water, which has a much higher N could not be used in this manner since the desired threshold temperature of approximately 200 F. lies below the temperature at which N for water peaks, which is 310 F., and thus below T for any water heat pipe.

Although the evaporator of the heat pipe 60 is shown within the exhaust manifold 64, various modifications may be employed, such as positioning of the evaporator ends of the heat pipe within specially formed holes in the engine block or within the vehicle muffler. Of course, in. the modification where the evaporator sections are inserted directly into the block, special tooling for changes to existing designs would be required and this may be prohibitive. In any. case, this system provides for rapid vehicle compartment heating during engine heatup since the exhaust gases, even initially, are at a relatively high temperature, and further, this system provides automatic cutoff of the heat pipes as a means for auxiliary thermal energy transport when the heat pipe temperature reaches the threshold temperature for the particular working fluid. The heat pipes 60 would be of a length of 3 feet or less and may be finned externally both at the evaporator and condenser ends for more efficient heat transfer. It is envisioned that the heat pipe would employ screen wire as the capillary structure. Further, the heat source (exhaust manifold) should preferably be at a lower elevation than the heat sink (heat exchange chamber 54) since it is desirable for the liquid phase flow to be aided by gravity rather than opposed by it.

There are two aspects of the invention in its general use. One uses a critical point heat pipe to maintain a heat sink above a given temperature. When the temperature of the heat sink falls toward this level the heat pipe temperature falls below the critical temperature and heat is transported from a heat source to the heat sink by the heat pipe to prevent the temperature from going lower. The other uses a critical point heat pipe to deliver heat to a heat sink as long as it is below a given temperature. The latter use corresponds to the specific environment shown. The heat pipe heat source is shut off when the heat sink (the compartment inlet air) rises above this temperature level.

While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method for exchanging heat between a heat source and a heat sink and for automatically discontinuing heat exchange when the heat sink reaches a desired reference temperature said method comprising:

a. thermally coupling said heat sink to said heat source by heat pipe means, and

b. employing vaporizable heat-pipe-working fluid having a critical point temperature substantially at the desired reference temperature for transporting heat from said heat source to said heat sink by means of said heat pipe when the temperature of said heat sink is below said reference temperature, and for automatically terminating heat exchange when the temperature of said heat sink reaches said reference temperature, said automatic termination of heat exchange being the direct result of the properties of said heat-pipeworking fluid.

2. In a system employing a heat pipe for thermally coupling a heat source to a heat sink and for exchanging heat between said heat source and said heat sink only when the temperature of said heat sink is different from a desired reference temperature, the improvement comprising:

a. a vaporizable working fluid carried by said heat pipe means having a critical point temperature substantially corresponding to the desired reference temperature for transporting heat from said heat source to said heat sink by means of said heat pipe when the temperature of said heat sink is below said reference temperature, and for automatically terminating heat exchange when the temperature of said heat sink reaches said reference temperature, said automatic termination of heat exchange being the direct result of the properties of said heat pipe working fluid.

3. An improved automotive heating system including a pas senger compartment and an internal combustion engine, said system comprising:

a. a heat exchanger,

b. means for circulating compartment air through said heat exchanger,

c. means for directing engine coolant from said internal combustion engine to said heat exchanger for heating said circulated air under normal operating conditions,

d. heat pipe means thermally coupling said engine exhaust gases and said circulated air downstream of said heat exchanger, and

e. a vaporizable working fluid carried by said heat pipe means to transport heat from said exhaust gases to said circulated air and having a critical point temperature corresponding substantially to the normal operating temperature of the engine coolant flowing through said heat exchanger, said heat pipe means for transporting heat from said heat source to said heat sink when the temperature of said heat sink is below said reference temperature and for automatically terminating heat exchange when the temperature of said heat sink reaches said reference temperature, said automatic termination of heat exchange being the direct result of the properties of said heat-pipe-working fluid.

4. The heating system as claimed in claim 3 wherein the evaporator portion of said heat pipe means is operatively coupled to the exhaust manifold of the internal combustion engine and the condenser portion of said heat pipe means is located at the downstream side of said heat exchanger.

5. The system as claimed in claim 4 further including insulating means carried by said heat pipe means intermediate of the evaporator and condenser portions.

6. The system as claimed in claim 4 wherein said vaporizable working fluid comprises freon.

7. The system as claimed in claim 4 wherein said vaporizable working fluid comprises ammonia.

8. The system as claimed in claim 4 wherein said heat exchanger is positioned at a higher elevation than said engine exhaust manifold.

9. The system as claimed in claim 3 wherein said heat-pipeworking fluid comprises one material of the group consisting of ammonia and freon and said heat pipe means includes a heat pipe tube formed of one material of the group consisting of copper, nickel, stainless steel, and aluminum. 

1. A method for exchanging heat between a heat source and a heat sink and for automatically discontinuing heat exchange when the heat sink reaches a desired reference temperature said method comprising: a. thermally coupling said heat sink to said heat source by heat pipe means, and b. employing vaporizable heat-pipe-working fluid having a critical point temperature substantially at the desired reference temperature for transporting heat from said heat source to said heat sink by means of said heat pipe when the temperature of said heat sink is below said reference temperature, and for automatically terminating heat exchange when the temperature of said heat sink reaches said reference temperature, said automatic termination of heat exchange being the direct result of the properties of said heat-pipe-working fluid.
 2. In a system employing a heat pipe for thermally coupling a heat source to a heat sink and for exchanging heat between said heat source and said heat sink only when the temperature of said heat sink is different from a desired reference temperature, the improvement comprising: a. a vaporizable working fluid carried by said heat pipe means having a critical point temperature substantially corresponding to the desired reference temperature for transporting heat from said heat source to said heat sink by means of said heat pipe when the temperature of said heat sink is below said reference temperature, and for automatically terminating heat exchange when the temperature of said heat sink reaches said reference temperature, said automatic termination of heat exchange being the direct result of the properties of said heat pipe working fluid.
 3. An improved automotive heating system including a passenger compartment and an internal combustion engine, said system comprising: a. a heat exchanger, b. means for circulating compartment air through said heat exchanger, c. means for directing engine coolant from said internal combustion engine to said heat exchanger for heating said circulated air under normal operating conditions, d. heat pipe means thermally coupling said engine exhaust gases and said circulated air downstream of said heat exchanger, and e. a vaporizable working fluid carried by said heat pipe means to transport heat from said exhaust gases to said circulated air and having a critical point temperature corresponding substantially to the normal operating Temperature of the engine coolant flowing through said heat exchanger, said heat pipe means for transporting heat from said heat source to said heat sink when the temperature of said heat sink is below said reference temperature and for automatically terminating heat exchange when the temperature of said heat sink reaches said reference temperature, said automatic termination of heat exchange being the direct result of the properties of said heat-pipe-working fluid.
 4. The heating system as claimed in claim 3 wherein the evaporator portion of said heat pipe means is operatively coupled to the exhaust manifold of the internal combustion engine and the condenser portion of said heat pipe means is located at the downstream side of said heat exchanger.
 5. The system as claimed in claim 4 further including insulating means carried by said heat pipe means intermediate of the evaporator and condenser portions.
 6. The system as claimed in claim 4 wherein said vaporizable working fluid comprises freon.
 7. The system as claimed in claim 4 wherein said vaporizable working fluid comprises ammonia.
 8. The system as claimed in claim 4 wherein said heat exchanger is positioned at a higher elevation than said engine exhaust manifold.
 9. The system as claimed in claim 3 wherein said heat-pipe-working fluid comprises one material of the group consisting of ammonia and freon and said heat pipe means includes a heat pipe tube formed of one material of the group consisting of copper, nickel, stainless steel, and aluminum. 